KENNEDY SPACE CENTER, Fla.--Despite threatening weather, NASA managers Wednesday cleared the shuttle Discovery for launch Thursday on its 39th and final mission, deciding an electrical glitch that prompted a 24-hour delay was not a threat to flight safety. Liftoff was targeted for 3:29:43 p.m. EDT.

NASA's mission management team reviewed an engineering assessment that concluded the electrical problem was most likely caused by "transient contamination" in a circuit breaker. Critical equipment on that circuit operated normally after the breaker was cycled several times and there have been no other signs of trouble.

"At the end of the day, I'll kind of cut to the chase and say we wrapped up with a unanimous poll out of the MMT, no dissenting opinions, no requests for additional data, everybody was very comfortable with the story that came together today," said Mike Moses, chairman of the mission management team.

"Really, the rationale there is all the evidence points to this being a circuit breaker problem, a power supply feed problem, it's not a controller problem so it's not a main engine controller. So no worries there the main engine controller itself is going to have problems later on."

A time exposure captures rollback of the rotating service structure, exposing Discovery to view. (Photo: NASA/Bill Ingalls)

While Discovery appears to be in good shape, dismal weather is on tap Thursday with high winds expected Friday and Saturday.

With a frontal system moving through the area, forecasters are predicting an 80 percent chance of low clouds and rain that would prohibit a launching Thursday. High winds are expected Friday and Saturday, with forecasters putting the odds of acceptable weather at 60 percent and 40 percent respectively.

The shuttle's current launch window runs through Sunday. After that, the angle between the sun and the plane of the station's orbit would result in higher-than-allowable heating of the shuttle after docking. The year's final shuttle launch window opens Dec. 1 and closes five days later.

Engineers were cleared to roll a protective gantry away from Discovery late Wednesday, after a delay because of a lightning advisory, setting the stage for the start of fueling at 6:04 a.m. Thursday.

Mission managers will meet at 5:30 a.m. to assess the weather.

"The weather still looks really bad for tomorrow," Moses said. "But we're going to go ahead and go down to the tanking telecon in the morning. ... If the forecast tomorrow morning is still as bad as it is today, there's a chance we might decide not to spend one of our (launch) opportunities tomorrow. But it's too early to make that call right now."

Assuming an on-time launch, Lindsey will guide Discovery to a docking with the International Space Station around 11:29 a.m. on Nov. 6. Spacewalks by Kopra and Drew are scheduled for Nov. 8 and 10. If all goes well, Discovery will undock from the lab complex around 5:27 a.m. on Nov. 13, setting up a landing back at the Kennedy Space Center at 10:24 a.m. on Nov. 15.

Cockpit instrument panel L4, located above and behind the commander's seat, where a suspect main engine contoller power circuit breaker is located (Photo: NASA)

Engineers started Discovery's final countdown Monday after working through the weekend to replace and retest leaking quick-disconnect fittings in the shuttle's right-side orbital maneuvering system rocket pod. That work forced NASA to delay launch from Monday to Tuesday and, eventually, to Wednesday.

The wiring behind panel L4. (Photo: NASA)

Launch was delayed 24 hours to Thursday after engineers ran into problems with a circuit associated with a control computer mounted on one of Discovery's three hydrogen-fueled main engines. One of the controller's two channels did not power up when a switch was thrown. An hour and 45 minutes later, the controller powered up on its own.

Engineers then powered down the controller and cycled the circuit breaker five times. They also turned the controller off and on. A switch was then cycled five times and then re-powered the controller. There were no problems.

An hour and 45 minutes after that, however, telemetry indicated a 5-volt drop in the circuit for a brief 180 milliseconds. The drop was within design specification, but it was unusual and mission managers ordered a 24-hour delay to gain a better understanding of the overall issue.

As it turned out, Moses said, the engineering analysis showed that short voltage drops are not unusual and that very slight changes in the operation of other equipment on the circuit can produce similar signatures.

"Another big piece of data that got us comfortable today was the fact that it's not all that uncommon," he said. "And so again, when we laid it all out it all racked up to be pretty clear that our most probable cause here is we had contamination on that circuit breaker, that we slowly cleared it off by scrubbing (cycling) it. In fact, after we left last night, we scrubbed it five more times. Those power-up signatures were perfect.

"So, pretty good proof that we knocked the contamination off. Our history shows us that once we do that, that is a solid connection and it's not going to change and therefore we had pretty good acceptance rationale today to go fly."

Each main engine is equipped with a controller that monitors engine operation -- valve positions, temperatures, pressure, vibration and other factors -- 50 times per second. Those data are fed to the shuttle's flight computers and if a problem develops, an engine can be safely shut down before a catastrophic failure occurs.

If a controller channel fails before launch, a countdown hold or an on-pad abort would result. If a controller channel fails after liftoff, the affected engine would continue operating with a single channel. A second failure, however, would trigger an automatic engine shutdown and abort.

But the engineering review determined the electrical anomalies in the backup channel of main engine No. 3's controller were most likely the result of transient contamination in a circuit breaker located in the shuttle's cockpit behind the commander's seat.

"Given history of contamination issues in circuit breaker operation, signature of failure seen in this launch countdown is most probable cause," according to an MMT presentation.

"System behavior following last circuit breaker cycling indicates a strong conducting contact surface across all three phases."